The administration of CPAP is common in the treatment of Obstructive Sleep Apnea (OSA) syndrome and Upper Airway Resistance syndrome. It has been postulated that CPAP treatment effectively acts as a pneumatic splint of a patient's upper airway by providing air or breathable gas at a pressure elevated above atmospheric pressure to the entrance of the patient's airway. Treatment pressures in the range 4-25 cm H.sub.2 O are commonly encountered.
Common to all forms of CPAP apparatus is a mask worn by a patient having connection via a flexible air delivery tube to a flow generator. Most often, the flow generator is driven by an electric motor that is under the control of a motor controller. In this specification reference to a "mask" is to be understood as including a nose mask, a mouth mask, a nose and mouth mask in combination, nasal prongs or nasal pillows, or a full face mask.
CPAP treatment can be in a number of forms, including (i) the maintenance of a constant treatment pressure level, (ii) alternating between two constant levels in synchronism with the inspiratory and expiratory phases of respiration ("bi-level CPAP"), and (iii) having an autosetting level in accordance with a patient's therapeutic needs. In all of these cases there must be control over the pressure of air or breathable gas supplied to the patient's airway.
In one form in the prior art, control over the treatment pressure is achieved by speed control of the electric motor driving the turbine (or fan) that together constitute the flow generator. In the case of bi-level CPAP, the motor must be able to accelerate (or decelerate) respectively to double (or half) its operational speed within about 100 ms. For typical CPAP treatment, this equates to the need to supply (or sink) approximately twice the steady state electrical power within the noted time interval. Disadvantages in motor performance associated with the rapid transitions in speed are, for example, noise due to magnetostrictive effects and bearing vibration, and increased thermal dissipation requirements. Lower noise will increase patient compliance with the treatment.
FIG. 1 shows, as a cross-sectional view, a conventional flow generator 10 comprising a chamber 12 that is segregated from the casing 14 of the CPAP apparatus. The casing 14 houses the control circuitry (not shown) associated with the flow generator 10. The flow generator further is comprised by a motor 16 driving an induced flow centrifugal turbine (impeller) 18, which induces the flow of air or breathable gas by an air inlet 20 to pass the air or breathable gas under pressure by an air outlet 22 to the air delivery tube (not shown) and so to the mask (also not shown). The turbine 18 has radially directed impeller blades 24. The alternate use of axial fans is known also in CPAP apparatus.
Another form of controllable flow generator involves operation of the driving motor at a constant speed, and venting or bleeding-off excess air from the output side of the turbine. As shown in FIG. 2, the turbine 18 is connected to a plenum chamber 30 by a supply pipe 32. The plenum chamber has a controllable spill valve 34 operable to indexingly open and close an opening 36 in the chamber wall to allow the venting of air to atmosphere so as to achieve the desired output pressure at the air outlet 38.
Such an arrangement also has disadvantages. Firstly there is excessive noise due to the venting of air when the treatment pressure is adjusted. This is particularly the case for each expiratory event during bi-level CPAP treatment when the treatment pressure typically is reduced from 16 cm H.sub.2 O to 6 cm H.sub.2 O, and thus over one half of the pressure head of the air within the plenum chamber 30 must be vented by the spill valve 34. It is also difficult to maintain precise treatment pressure regulation, since small variations of the spill valve position give relatively large variations in the pressure at the air outlet 38. This configuration also leads to an inherently low maximum flow rate which can compromise the efficacy of CPAP treatment. In particular, the spill valve 34 works by increasing outlet flow from the plenum chamber 30, thereby increasing the pressure drop in the supply pipe 32 and the turbine 18, thus dropping the pressure in the plenum chamber. The combined pneumatic impedance of the supply pipe 32 and the turbine 18 limit the maximum achievable flow rate into the plenum chamber 30, and so to the patient, on subsequent closure of the spill valve 34.
An example of another prior art arrangement that operates on the output of the flow generator can be obtained from International Publication No. WO 90/14121 (PCT/US90/02800), in the name Puritan-Bennett Corp.
As is noted, the invention also has application to apparatus for the provision of assisted respiration. Use of the term "assisted respiration" is to be understood as embracing both ventilators and respirators. Ventilators can broadly be characterised as providing for patient ventilation in a volume cycled mode, and do the work of breathing for the patient. Respirators, on the other hand, may or may not do the complete work of breathing for a patient, and are characterised by their bi-level operation, with a large treatment pressure differential between inspiration and expiration and a high inspiratory treatment pressure, which may reach 30-40 cm H.sub.2 O.